The present invention relates to a vaneless diffuser type of centrifugal compressor and pump.
Vaneless diffusers of centrifugal compressors as generally known in the art are useful for refrigerant pumping purposes in an air-conditioning system, such as those on-board U.S. Naval marine vessels. A conventional type of centrifugal compressor/pump having a vaneless diffuser includes a power driven impeller through which inflow of the refrigerant under suction pressure is induced for radially outward inflow into the vaneless diffuser from which outflow of the refrigerant is delivered for discharge. The vaneless diffuser in such a centrifugal compressor maybe of the annular passage type, wherein a wall surface of a fixed plate is axially spaced from a shaped wall surface of a shroud to form a radial flow passage having a lower inlet end receiving the impeller outflow and a radially outer outlet end from which outflow occurs into a discharge passage of the compressor volute that is circumferentially divergent for example. Fluid kinetic energy is converted by such diffuser of the compressor into a static-pressure rise in the refrigerant by convergent passage flow from the passage inlet end toward the exit portion of the passage at its outlet end. Flow separation from the wall surfaces of the diffuser passage occurs, dependent on the fluid pressure rise to adversely affect operational stability and efficiency.
It is therefore an important object of the present invention to improve operational stability and efficiency of the foregoing type of compressor by achieving higher pressure recovery and lower non-recovery losses for the entire compressor operating range.
In accordance with the present invention, the diffuser shroud surface of a centrifugal compressor is contoured to provide for more efficient energy transfer during flow of fluid through a vaneless diffuser between its fixed surface and the shroud surface. The shroud surface contouring involves establishment of a shroud surface profile providing continuously converging flow passage from its inlet end to a location at a pinch point at which a minimum passage area is established. A divergent portion of the flow passage formed by the surface profile extends from the pinch point location to a location from which outflow completes the static fluid pressure rise for discharge from the diffuser outlet end into the volute portion of the compressor. Such outflow is effected at an exit angle less than that of the inflow convergence angle of the passage profile from the inlet end so as to accommodate a smooth diffuser outflow into the discharge passage formed in a volute portion of the compressor.
Contouring of the shroud surface profile is performed by optimizing calculations at plural locations along the fixed diffuser passage surface, based on diffuser and volute flow predictions. The procedure for such calculations based on flow predictions is set forth in two publications of Y. T. Lee et al. consisting of an article published in 1998 in xe2x80x9cInternational Journal of Rotating Machineryxe2x80x9d Vol. 5, No. 4, entitled xe2x80x9cPerformance Evaluation of an Air-Conditioning Compressorxe2x80x9d on pages 241-250 and an article presented in the International Gas Turbine and Aeroengine Congress and Exhibition, held in Munich, Germany during May 8-11, 2000, such article being entitled xe2x80x9cDirect Method for Optimization of a Centrifugal Compressor Vaneless Diffuserxe2x80x9d.